As the communication devices are getting smaller and smaller to comply with the design trend of lightweight, thinness and compactness, the antenna structures disposed on the communication devices also need to be miniaturized. However, when most antenna structures are multi-input multi-output (MIMO) antennas, and several antennas are disposed within a limited planar area, it is inevitable that signal interference will occur between antennas. Therefore, how to reduce signals interference between antennas or increase the isolation between antennal signals has become a prominent task for the industries.